in gen 5, if a male pokemon holds an everstone in breeding, there is still a 50% chance of having the same nature, correct?
No, only the female can pass down the nature w/Everstone.
This is incorrect. While it was true for Emerald and DPPt, the effect was changed in HGSS from only working with females to working with both males and females:
Bulbapedia Evolutionary Stone Page said:If held in the Pokémon Day Care by a female Pokémon or Ditto, Pokémon bred will have a 50% chance of having the nature of the holder.EDPPt
If held in the Pokémon Day Care by either parent, Pokémon bred will have a 50% chance of having the nature of the holder.HGSSBW



